Therapeutic duplication of prescriptions is the most frequently reported inappropriate drug use in Korea. To prevent significant problems during drug prescribing and dispensing, prospectively, development of standard including drug lists considered as therapeutic duplications for the prioritized drug classes first would be necessary. This study was aimed to analyze frequent drug classes of therapeutic duplications by healthcare providers in clinical practice settings. National health claims data for drug review and reimbursement (1,426,065 prescriptions dated March 19, 2008) were analyzed. Therapeutic duplication was defined as the prescription including more than 2 ingredients belonging to the same KFDA drug classification numbers that considered to have therapeutic similarities. The following 3 drug classes were mostly frequent therapeutic duplication classes: 114 anti-pyretics, analgesics and anti-inflammatory drugs; 117 drugs for psycho-nervous system; 141 Antihistamines. About 3.5% of overall prescriptions analyzed showed therapeutic duplications. This result might be starting step to develop DUR therapeutic duplication standard.